I just use a Paterson grain focuser - it is simple, cheap, robust and adjustable. I have a Hocus Focus but find it is really only good for rough focusing (I do need reading glasses however, so if you have good eyesight it may be sufficient). I switch to the grain focuser for a final tweak of the focus. It is quite tough: made from a single bent strip of perspex with a mirror glued on, so there is not much to break.

Cheers, Bob.